msmtp: disable libidn support

msmtp: disable libidn support

When libidn has been populated to sysroot but rpm packages are not
created, build msmtp will check libidn support automatically and then
depends on libidn. But package libidn doesn't exist, build image which
includes msmtp will fail with:

| error: Failed dependencies:
| 	libidn11 >= 1.25 is needed by msmtp-1.4.24-r3.i586
| 	libidn.so.11(LIBIDN_1.0) is needed by msmtp-1.4.24-r3.i586
| 	libidn.so.11 is needed by msmtp-1.4.24-r3.i586

Disable libidn support to fix this issue.

(From OE-Core rev: a304789cecb1a6ffa3c5a56f617eea4a5ac29a98)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>